The present invention relates to a wavelength-selective polarization conversion element that generates two light components having polarization directions and wavelength regions different from each other from light entering thereinto, an illumination optical system equipped therewith and an image projection apparatus.
A so-called three-panel projector (image projection apparatus) performs a color-separation to separate white light emitted from a light source into three color light components (for example, a red light component, a green light component and a blue light component). And then, the color light components are introduced to three image-forming elements such as liquid crystal panels.
Japanese Patent Laid-Open No. 2000-9933 discloses the following three-panel projector. That is, in order to perform color-separation through a color separation optical system, the three-panel projector is provided with an illumination optical system. The illumination optical system generates two wavelength components having polarization directions different from each other (for example, P-polarized green light and S-polarized red and blue light) from non-polarized white light emitted from a light source.
The illumination optical system separates the non-polarized white light emitted from the light source into S-polarized red and green light and P-polarized blue light with a polarization splitting surface of a polarization beam splitting prism. And further, a half wave plate converts the P-polarized light into S-polarized light. The S-polarized red, green and blue light impinges on a polarization conversion dichroic mirror. The polarization conversion dichroic mirror converts only the S-polarized green light into P-polarized light. Thus, the P-polarized green light and the S-polarized red and blue light proceed to the color separation optical system.
The polarization conversion dichroic mirror includes a dichroic surface, a quarter-wave plate and a mirror surface in this order from a light entrance side. The dichroic surface transmits the S-polarized green light, and reflects the S-polarized red and blue light. The S-polarized green light passes through the dichroic surface, and passes twice through the quarter-wave plate before and after the reflection by the mirror surface. Thereby, the quarter-wave plate converts the S-polarized green light into P-polarized green light. With this arrangement, the P-polarized green light and the S-polarized red and blue light emerge from the polarization conversion dichroic mirror.
However, in the illumination optical system disclosed in the Japanese Patent Laid-Open No. 2000-9933, the red light component, green light component and blue light component entering into the polarization conversion dichroic mirror have to be previously converted into S-polarized light by the polarization beam splitting prism and the half wave plate. Guiding the light from the light source to the polarization conversion dichroic mirror via the polarization beam splitting prism and the half wave plate as described above decreases the use efficiency of the light. Also, since the configuration in which the light from the light source is reflected via the polarization beam splitting prism and the polarization conversion dichroic mirror is employed, the designing flexibility of the illumination optical system is restricted and the size of the illumination optical system tends to become larger.